For conventional exhaust gas recirculation (EGR), exhaust gas expelled from all of the cylinders of an internal combustion engine may be collected in an exhaust manifold. A fraction of the collected exhaust gas (e.g. 5% to 10%) may then be routed from the exhaust manifold through a control valve back to an intake manifold of the engine, where it may be introduced to a stream of fresh (ambient) intake air. The remaining fraction of exhaust gas in the exhaust manifold, rather than being recirculated and recycled, generally flows to a catalytic converter of the exhaust system and, after treatment therein, may be expelled to the atmosphere.
EGR has a history of use in gasoline spark-ignition engines, and affects combustion in several ways. First, the combustion in the cylinders of the engine may be cooled by the presence of exhaust gas, that is, the recirculated exhaust gas may absorb heat from the combustion. Furthermore, the dilution of the oxygen present in the combustion chamber with the exhaust gas, in combination with the cooler combustion, may reduce the production of mono-nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide (NO2). Additionally, EGR may reduce the need for fuel enrichment at high loads in turbocharged engines and thereby improve fuel economy.
EGR which uses higher levels of exhaust gas may further increase fuel efficiency and reduce emissions of spark-ignition engines. However, with higher levels of exhaust gas, engines may face challenges related to EGR control and tolerance, which may reduce the expected fuel efficiency improvement. Challenges related to EGR control may be understood to include reducing variability of the exhaust gas, particularly composition and distribution. If the variation in the exhaust gas introduced to an engine is too random, fuel efficiency improvements may suffer. Challenges related to EGR tolerance may be understood to include increasing an engine's ability to process higher levels of exhaust gas without adversely affecting performance, particularly fuel economy. Thus, even if EGR control and tolerance may be satisfactory for engine operation at low levels of EGR, an engine may need additional modifications in structure and operational conditions to accommodate higher levels of EGR without adversely affecting engine performance.
More recently, an engine configuration has been proposed with one or more cylinders of the engine are dedicated to expelling exhaust gas for EGR, which is then directed to the intake manifold. Such cylinders may be referred to as dedicated EGR, or D-EGR, cylinders. Dedicated EGR cylinder(s) may operate at a broad range of equivalence ratios since their exhaust gas is generally not configured to exit the engine before flowing through a cylinder operating at, for example, a stoichiometric or near stoichiometric air/fuel ratio. This may allow the dedicated EGR cylinder to be run rich to produce higher levels of carbon monoxide (CO) gas and hydrogen (H2) gas which, may in turn, increase the octane number and promote increased EGR tolerance and knock tolerance by increasing flame/speed burn rates, as well as increasing the dilution limits of the mixture and associated combustion stability of all the cylinders. Examples of engines with a D-EGR cylinder may be found in U.S. Patent Application Publication No. 2012/0204844 entitled “Dedicated EGR Control Strategy For Improved EGR Distribution And Engine Performance” and U.S. Patent Application Publication No. 2012/0204845 entitled “EGR Distributor Apparatus For Dedicated EGR Configuration”, both in the name of Jess W. Gingrich, which are assigned to the assignee of the present disclosure and hereby incorporated by reference to the extent they are consistent with the present disclosure.
In order to further increase hydrogen (H2) gas production, both of the preceding Gingrich applications disclose that the carbon monoxide (CO) gas of the exhaust gas from the D-EGR cylinder may react with water (H2O) vapor using a water gas shift (WGS) reaction with a suitable water gas shift (WGS) catalyst. With the WGS reaction, carbon monoxide (CO) gas in the exhaust gas may react with water vapor to produce carbon dioxide (CO2) gas and hydrogen (H2) gas. Reacting carbon monoxide (CO) gas in the exhaust gas with water vapor to produce hydrogen (H2) gas may be understood as being beneficial by increasing the amount of hydrogen (H2) gas in the exhaust gas from dedicated EGR cylinder.
However, while a water gas shift catalyst may result in production of hydrogen (H2) gas, the water-gas shift catalyst exchanges carbon monoxide (CO) gas for hydrogen (H2) gas and carbon dioxide (CO2) gas, meaning that any hydrogen (H2) gas produced results in a loss of combustible carbon monoxide (CO) gas and a gain of incombustible carbon dioxide (CO2) gas, which does not provide the same level of combustion benefit as hydrogen (H2).
Furthermore, the water-gas shift reaction is mildly exothermic, meaning energy is released as the reaction progresses, and thus energy is lost through the process. More particularly, for an engine system with a traditional low level EGR loop, a heat exchanger is generally utilized to reduce the temperature of the EGR feed prior to mixing with intake air. This heat is typically rejected to the engine coolant, and is then subsequently rejected to the ambient environment via the radiator. Similarly, in a gasoline D-EGR application, the energy released during the exothermic water-gas shift reaction is rejected via the same process, meaning that the energy produced by the reaction is not used in a useful manner. As a result, it may be understood that the energy content of hydrogen (H2) gas created is slightly less than the energy content of the carbon monoxide (CO) gas consumed. Consequently, an alternative to use of a water gas shift reaction to generate hydrogen (H2) gas is needed which does not suffer from the foregoing drawbacks.
Furthermore, it is desirable that the alternative to the water gas shift reaction be applicable to engines which utilize natural gas as a fuel, in addition to more traditional gasoline. While gasoline may be the fuel most recognized for D-EGR engines, recent advancements in shale fracking and horizontal drilling techniques have vastly improved the yields of natural gas wells in North America. This rapid increase in production has resulted in a substantial drop in the cost of natural gas, which is primarily formed of methane (CH4), in the United States. This substantial and rapid drop in the cost of natural gas has resulted in renewed interest in natural gas fueled motor vehicles with improved efficiencies. The use of exhaust gas recirculation (EGR) in hydrocarbon based automotive engines has the potential to further improve the efficiency of these engines.
Moreover, it is desirable that the alternative to the water gas shift reaction not be limited to D-EGR engines and be applicable to engines other than a D-EGR engine.